Wind turbines are rapidly becoming a significant source of electrical power throughout the United States and in the world. A typical wind turbine consists of an electrical generator mounted in a nacelle atop a tower that may be 60 or more meters tall. The generator is powered by wind which rotates turbine blades. The turbine blades, which may be up to 50 or more meters in length and weigh upwards of 1900 kg, are mounted to a hub at the forward end of the nacelle. Typically, three blades are used, and a gearbox converts the relatively slow turbine revolution rate to a faster rate of revolutions suitable for the generator.
As with most mechanical machinery, periodic maintenance, refurbishing or repair is needed for wind turbines, including the occasional need to repair or refurbish a blade of the wind turbine. When such refurbishing or repair cannot be administered upon the blade while attached to the hub, it is necessary to remove the blade from the wind turbine and lower it to the ground, where the necessary work may be performed. Upon completion of the work, the blade must be raised to the nacelle and reattached to the hub. The extreme length and weight of a blade, and its aerodynamic shape and structural characteristics, make the job of lowering and raising a blade from a turbine in the field a difficult and exacting task. In addition, blades must be removed from wind turbines situated in areas in which accessibility is limited, and in which the local terrain may not permit the ingress or egress of larger vehicles, such as large cranes. Moreover, because wind turbines are designed to be situated in areas of more-or-less constant prevailing winds, the winds have a tendency to interfere with the raising or lowering of a turbine blade, threatening to knock it against the adjacent tower or to cause the blade to twist and rotate during the lifting process.
Various systems have been developed for performing the task of raising and lowering wind turbine blades for maintenance, refurbishing, or repair. One of these systems is U.S. Pat. No. 7,726,941, Ser. No. 11/738,685 (Pub. No. US-2007-0266538) to Bervang, which describes a method for raising a turbine blade using a rigid yoke that grips the blade to be raised or lowered with a jaw-like structure. The yoke has an integral counterweight which assists in rotating the blade between vertical and horizontal positions, and is raised or lowered by a crane. Tag lines between the yoke and the crane are used to turn or otherwise position or manipulate the blade during raising or lowering. Although this system uses tag lines to prevent the blade from twisting or banging against the tower, its use of a crane hoisting a rigid yoke with a jaw-like structure places extreme pressure point stresses upon interior portions of the blade and requires a suitably smooth or flat area in the vicinity of the base of the tower to support a large crane.
U.S. Pat. No. 8,083,212 discloses a lifting system in which a lifting winch may be mounted in the rotor head, in the nacelle, or on the ground, and that lifts a turbine blade with a cable attached to the winch. A movable sheaf, or pulley, supports a hoisting cable from within the nacelle. A frame structure serves as a harness or yoke to grasp the blade at or near the root, and maintains it in a desired vertical or horizontal orientation as it is suspended from the cable. As the blade is lifted, the frame creates forces on the blade that cause the blade to assume a particular angle with reference to the vertical. Although the harness may be effective in controlling the vertical and horizontal orientation of the blade, this system does not have tag lines that would be necessary to prevent the wind from blowing the blade against the tower or to prevent it from twisting during the hoisting operation.
Canadian Patent CA 2,692,705 to Reed et al. discloses a system in which two winches mounted at ground level on either side of a wind turbine tower each operate a hoisting cable. Each cable extends upward to a pulley that is attached to the rotor hub, which then extends downward to a blade that is to be hoisted up to the hub. A pick crane assists the process by lifting the lower tip of the blade. The use of two cables, one at either side of the blade, provides some protection against the blade's becoming twisted in light winds during hoisting. However, without having taglines to brace the blade against twisting in higher winds, and to hold the blade away from the tower during such winds, this system is limited to being used only in no wind or light wind situations—which are the antithesis of optimal placement for a wind turbine farm.